The Darkness Beneath by Keri Beevis

Lizzie Kent wasn’t supposed to babysit the night she was murdered. She was covering for her best friend, Nell O’Connor, so Nell could sneak out and meet her boyfriend. Nell has had to live with the guilt ever since.

Eighteen years later and Nell returns to Purity Island, desperate to escape a bad relationship. She has inherited her aunt’s rundown guesthouse and hopes the island will offer her sanctuary and a fresh start. Her return isn’t welcomed by everyone, though, in particular Sam Kent, who blames Nell for his sister’s death. A few unsettling incidents soon make it apparent that someone is trying to spook her.

Is Sam responsible or has Nell’s abusive ex-boyfriend managed to track her down? Or is it someone with a more sinister agenda?